The present invention relates to a method of fabricating a semiconductor laser device operable at a high temperature stably for a prolonged time.
A conventional method of fabricating such a device will be described with reference to FIG. 1. The device shown in FIG. 1, which a so-called "buried crescent" (BC) structure, is composed of an active region 12, which is crescent shaped in cross section, and a current limiting mechanism for concentrating a current flow.
On an n-type InP substrate 1, a p-type InP layer 2 and an n-type InP layer 3 are grown in the stated order by LPE (Liquid Phase Epitaxy) using a saturated solution containing In as a solvent and InP as a solute. Then, a groove 60 having width of 2 microns and depth reaching the substrate 1 is formed by a combination of photolithography and etching. Thereafter, an n-type InP region 11, an active region 12 of p-type InGaAsP or n-type InGaAsP, a p-type InP region 13, and a contact layer 14 of p-type InGaAsP are formed in the stated order by LPE using a saturated solution containing In as a solvent and InP, GaAs, InAs, and dopants, etc., as solutes. The position of the active region 12 is controlled such that opposite ends thereof are in contact with the p-type InP layer 2. Then, an n-type electrode 21 and a p-type electrode 22 are formed on opposite surfaces of the wafer.
In the conventional device fabricated as described above, a p-n junction through which a current flows includes a p-n junction portion which is in contact with the active region 12 and through which a current arising due to a laser oscillation flows, and a p-n junction 50 of InP extending on both sides thereof. The current flowing through the p-n junction 50 does not participate in the laser oscillation function, and therefore it should be as small as possible. The n-type InP region 3 which is buried in the p-type InP region 13 forms a potential barrier against current flow and therefore acts as a slit for concentrating the current to the active region 12.
In order to obtain efficient laser oscillation, it is necessary to concentrate the current in the active region 12. For this purpose, such a slit is effective. However, since the current diverges after passing through the slit, the use of such a slit is not sufficient. The p-n junction of the active region 12 is an InP - InGaAsP heterojunction, or, when the junction is within the active region 12, an InGaAsP homojunction, and the p-n junctions 50 in both sides of the active region are InP homojunctions. There is a difference in junction potential between the p-n junction of the active region 12 and the p-n junctions 50 since the current flows, as defined by exp (aV.sub.i /kT) where V.sub.i, q, k and T are the junction potential, electron charge, Boltzmann's constant and absolute temperature, respectively, tend to flow through the p-n junction of the active region 12 rather than through the p-n junctions 50 because the junction potential of the latter is higher than that of the former.
That is, in a semiconductor laser device having a BC structure, a major portion of the current flows concentrate in the active region due to the existence of the two kinds of p-n junctions to thereby obtain efficient laser oscillation. In other words, the junction potential of the p-n junction is determined by parameters of the material and the degree of doping.
If these factors are well selected, the junction potential difference between the p-n junction of the active region 12 and the p-n junction 50 will be a value, for instance, about 0.3 V, which is sufficient to obtain an acceptably great current concentration in the active region. However, if the junction potential difference is lowered due to incompleteness of crystallization, etc., the ineffective current is increased, causing the laser characteristics to be degraded. The more pronounced is this phenomenon, the higher the temperature since the electron energy distribution is broadened. Thus, the laser oscillation characteristic is degraded at high temperatures.
Particularly, for the p-n junctions 50 of InP, the p-n junction in sidewall surfaces of the groove 60 is formed during the second crystal growth for which the surfaces are exposed to a high temperature environment. Therefore, the junctions tend to include many crystal defects, and thus the junction potential of these junctions tends to be lowered. Further, the reduction of junction potential may occur when the laser is operated for a prolonged time for which a current flowing through the active region becomes large enough to degrade the latter, resulting in laser characteristics degradations such as increase of laser threshold value and fading out of oscillation at high temperature. The latter face has been confirmed experimentally by the inventors, as disclosed in "Position of the Degradation and the Improved Structure for the Buried Crescent InGaAsP/InP (1.3 micron) Laser", Applied Physics Letters, vol. 43, pp. 187-189, 15 July 1983.
It has been known that the degradation of laser characteristics mentioned above may be avoided to some extent by a heat treatment of the laser to diffuse in an impurity to thereby shift the p-n junction slightly from the interface exposed to the high temperature. FIG. 2 shows another conventional laser structure in which the above heat treatment is introduced.
In FIG. 2, an n-type InP layer 6 and a p-type InP layer 7 are grown in the stated order on a p-type InP substrate 5 and a groove 60 is formed as before. Then, in the second step, a p-type InP layer 15, an InGaAsP active region 16, an n-tupe InP layer 17, and n-type InGaAsP layer 18 are grown in the stated order on the wafer. At this time, the active region 16 is positioned such that it contacts the p-type InP layer 7. By heat treating the wafer during the second growth step, or heat treating only the p-n junction except for in the active region 16, the junction 50 is shifted in position. Reference numeral 30 depicts a p-type InP region.
With this structure, the p-n junction through which the current flows is separated from the interface exposed to the high temperature ambient, and thus a satisfactory junction characteristic is obtained.
In this conventional laser, however, it is necessary to increase the impurity concentration of either the p-type or the n-type region of the junction (usually, the p-type region since it is more easily moved) to about 10.sup.18 ions/cm.sup.3 or more, which reduces the effect of the slit for concentrating the current distribution. Further, because the junction includes regions containing crystal defects, the recombination rate of injected carriers is increased, resulting in a reduction of the junction potential.